1. Field
One or more aspects of embodiments according to the present invention relate to Fourier transform infrared spectrometers and in particular to methods for improving the operating speed of such spectrometers.
2. Description of Related Art
Imaging spectrometers are instruments which measure the spectrum of light received from a scene being observed, at each point in the observed image. In the case of an infrared spectrometer, much of this light may be thermally emitted radiation, with a blackbody envelope depending on the temperature of the emitting object, and with deviations from the blackbody spectrum at wavelengths at which the emissivity of the emitting surface is less than that of a blackbody. Such data may be useful for remote temperature measurements or for remotely identifying materials, for example.
An imaging spectrometer may be constructed from a Michelson interferometer, in which the length of one arm is scanned by moving a scan mirror. For a uniform scan rate the optical power at the output of the interferometer corresponds to the autocorrelation function of the incident light; taking the Fourier transform of the autocorrelation, for example using a fast Fourier transform (FFT) algorithm, results in the spectrum of the light. An instrument that operates in this manner is known as a Fourier transform spectrometer (FTS). A lens may be placed at the output of the interferometer, so that light from different source points in the scene is focused on different pixels of a detector, making it possible to obtain individual spectra for different points in the scene. Each exposure sampled by the detector is known as an interferogram, or sample.
In a spectrum obtained from a conventional FTS, the number of spectral bands obtained is less than or equal to N/2 where N is the number of detector samples obtained during the scan. The spectral resolution is given by 1/(2L), where L is the change in position of the scan mirror. For example, if the scan mirror moves 0.125 centimeters (cm), then the wave number separation between adjacent spectral bands will be 4 per cm (cm-1), where the wave number is the reciprocal of the wavelength, with a scaling factor of 1e4 if units of cm-1 and microns are used for wave number and wavelength, respectively. In this example, 150 samples would be required to cover the range of wavelengths between 7.4 and 13.5 microns, which corresponds to wave numbers ranging from 740 cm-1 to 1340 cm-1 (where, for example, the wave number corresponding to 7.4 microns is 1e4/7.4, or 1340, cm-1).
An uncooled microbolometer is an inexpensive infrared detector array fabricated, for example, for use in infrared cameras. For use in an imaging FTS, an uncooled microbolometer has the advantage that no components need to be cooled but the disadvantage, compared to other detector technologies, of a relatively low sample rate, which may be 60 frames per second. At this rate, obtaining 150 samples, for example, will take 2.5 seconds, and, in this interval, the scene may change, resulting in ambiguous data. There is a need, then, for a spectrometer to more rapidly collect scans while using uncooled microbolometer detectors.